This invention relates to a low cost, integrated, closed loop control system for providing efficient fuel utilization in induced draft, gas fired furnaces and boilers.
Power combustion (forced or induced draft) is used more and more frequently to increase the efficiency of gas fired furnaces and boilers that have either conventional or modified clam shell type heat exchangers. A prior art control system for forced draft furnaces and boilers is shown, by way of example, in U.S. Pat. No. 4,118,172.
With respect to induced draft combustion, in many existing boilers and furnaces the induced draft blower is located downstream of the heat exchanger and is used with an orifice, restricted flue passageway, or other similar device to produce a pressure drop which pulls the products of combustion from the combustion chamber into an existing chimney or into a through the wall exhaust pipe. Many of these existing systems use a single stage firing rate burner and an intermittent ignition device (IID). This in combination with a well designed heat exchanger and low off-cycle losses can provide Annualized Fuel Utilization Efficiencies (AFUE) in the range of 82-83%. However, such systems are costly. For example, code requirements in most locations dictate that such units incorporate one or two pressure switches to sense proof of combustion air, and a condition of a blocked stack.